Stimulation of selected nerves in the human nervous system by electrical signals is a known technique in assisting accident or disease victims who have lost normal neuromuscular control. The resulting paralysis is often seen in spinal-injury victims who lose voluntary control of the entire lower body. Injury or disease of this type interrupts the normal control mechanisms of the brain and spinal cord, leading to malfunctioning of motor and sensory systems, as well as other systems under nerve control below the point of injury or disease.
The nerve bundles and fibers below (toward the lower body) the injury site typically remain in good condition, but are nonfunctional due to the "open circuit" in the spinal cord. In theory, it should be possible to stimulate specific nerves below the point of injury or disease, and hence to activate the muscles, or other organs or systems controlled by those nerves, by applying electrical signals to electrodes surgically implanted on nerves emerging from the intact lower spinal cord. In practice, however, this goal is difficult to achieve because the spinal cord is a highly protected structure within the body, and conventional electrode-implantation surgical procedures are difficult, complex, and lengthy, thus presenting a relatively high degree of risk and trauma to the patient.
The spinal cord is surrounded by a tough tubular membrane serving as a protective sheath and called the dura mater (or simply "dura"). The dura is in turn protected and surrounded by the vertebrae which form the spinal column or backbone. The hollow interiors of the vertebrae form a vertebral canal through which the dura and cord extend, and the tissue- and vein-filled annular space between the wall of this canal and the dura is called the epidural space.
The space between the spinal cord (the surface of which is covered by a more delicate membrane called the pia mater) and the interior of the dura is filled with cerebrospinal fluid ("CSF") which is enclosed by the third of the cord-protecting meninges called the arachnoid. The dura extends substantially below the lower end or conus medullaris of the spinal cord. The lower end of the dura is filled with CSF and occupied by the lumbar and sacral nerves (each of which has a dorsal sensory root and a ventral motor or muscle-controlling root) and a bundle of smaller nerve fibers, all collectively called the cauda equina. These nerves are of great importance in that they control leg movement, as well as a number of lower body functions such as urination.
One way of reaching those nerves to enable artificial electrical stimulation involves a major surgical operation to expose the sacral nerves where they emerge from the dura. Electrodes can then be secured to the appropriate nerve bundles (depending on the function to be controlled), and the implanted electrodes are then energized by electronic devices which may be either internal or external to the patient's body. This kind of surgery is lengthy and difficult, and involves significant risk and recovery problems for the patient.
The electrode system of this invention overcomes many of these problems by enabling a greatly simplified surgical installation procedure. The new electrode is designed for introduction into the lower end of the dura beneath the conus of the spinal cord to float in the intrathecal (or "within the sheath") space loosely occupied by the sacral roots and other nerves of the cauda equina. Installation of the electrode is of little more complexity and risk than that presented by a conventional lumbar-puncture spinal tap for withdrawing a diagnostic sample of CSF from the subarachnoid space around the cauda equina. The electrode system is also useful in an extradural position to stimulate selected levels of nerves of the cauda equina through the dura.